The art of making cellular thermoplastic products is very well developed. It has long been known, for example in extrusion and molding processes, to form expanded thermoplastic products having a cellular structure by the injection of a gas under pressure into molten thermoplastic materials. Of the more recent patent literature, U.S. Pat. Nos. 2,928,130; 3,268,636; 3,436,446 and 3,975,473 are examples of such processes. These patents, however, are merely illustrative and it is not intended to suggest that they are the only or most pertinent background patents. Such patents disclose the widely used technique of injecting gas under high pressures into an extruder or barrel for solubilization in a thermoplastic melt, followed by discharge of the melt under reduced pressure thereby resulting in the production of foam. Commonly, the gas is introduced under pressures at least from about 500 psi to about 3000 psi into the extruder or molding apparatus. Thus, it is an objective in processes of this type to achieve immediate solubilization of the gas into the hot melt under pressure. There are variations in these known processes including the employment of additives, such as nucleating agents for the production of cellular thermoplastic bodies. Nucleating agents provide nucleating sites for the gas cell formation during the expansion or blowing of the molten thermoplastic materials.
Techniques are disclosed for making hot melt thermoplastic adhesive foams in U.S. application Ser. No. 710,377, filed Aug. 2, 1976 entitled "Hot Melt Thermoplastic Adhesive Foam System" now U.S. Pat. No. 4,059,714 and U.S. application Ser. No. 710,378, filed Aug. 2, 1976 for "Hot Melt Thermoplastic Adhesive Foam System" now U.S. Pat. No. 4,059,466, which applications are assigned to the assignee of this application. These applications are directed to methods of making and using hot melt foamed adhesives which offer significant improvements over conventional non-foamed adhesives. According to the techniques described in these applications, the gas is intimately mixed with a thermoplastic adhesive while the adhesive is in a molten state and then the mixture is pressurized to form a solution which, upon dispensing at a lower pressure, releases the gas and foamed adhesive results. The techniques described in these applications result in very useful foamed adhesives and bonding techniques. However, such techniques are not entirely satisfactory in producing hot melt thermoplastic adhesives on a continuous, uniform quality basis. Such techniques have also been found to be limited in their applicability to hot melt thermoplastic materials and further improvements are needed.